Microelectronic flip chip packages with solder wetting pads and associated methods of manufacturing

ABSTRACT

Processes of assembling microelectronic packages with lead frames and/or other suitable substrates are described herein. In one embodiment, a method for fabricating a semiconductor assembly includes forming an attachment area and a non-attachment area on a lead finger of a lead frame. The attachment area is more wettable to the solder ball than the non-attachment area during reflow. The method also includes contacting a solder ball carried by a semiconductor die with the attachment area of the lead finger, reflowing the solder ball while the solder ball is in contact with the attachment area of the lead finger, and controllably collapsing the solder ball to establish an electrical connection between the semiconductor die and the lead finger of the lead frame.

TECHNICAL FIELD

The present technology is directed to microelectronic flip chip packageswith solder wetting pads and associated methods of manufacturing.

BACKGROUND

Flip chip is a method for directly interconnecting semiconductor diesand a substrate with solder balls (or solder bumps). During assembly,solder balls are first deposited onto a semiconductor die. A solder maskis formed on the substrate (e.g., a printed circuit board) to define aplurality of connection sites. The semiconductor die with the solderballs is then flipped over to align the solder balls with correspondingconnection sites on the substrate. The solder balls are then reflowed tocomplete the interconnection.

One drawback of the foregoing flip chip technique is that the solderballs tend to uncontrollably collapse during reflow when the substrateis a lead frame having a plurality of lead fingers. The collapsed solderballs may cause various structural, functional, and/or other types ofdamages to the resulting microelectronic package. For example, adjacentsolder balls may come in contact with one another to short circuit thesemiconductor die and/or the substrate.

One conventional solution is to form a solder mask on the lead frame ason a printed circuit board. However, forming a solder mask on the smalllead fingers is difficult, time consuming, and costly. Accordingly,there is a need for improved flip chip techniques that can at leastreduce or eliminate the risk of uncontrollable collapse of solder ballsduring reflow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F are partially schematic, cross-sectional views of asemiconductor die and/or a portion of a lead frame undergoing a processto form a microelectronic package in accordance with embodiments of thetechnology.

FIGS. 2A-2E are partially schematic, cross-sectional or plan views of aportion of a lead frame undergoing a process in accordance withadditional embodiments of the technology.

FIGS. 3A-3C are partially schematic and cross-sectional views of aportion of a lead frame undergoing a process in accordance with furtherembodiments of the technology.

FIGS. 4A and 4B are partially schematic, cross-sectional views of aportion of a lead frame undergoing a process in accordance with yetadditional embodiments of the technology.

FIGS. 5A and 5B are cross-sectional images of chip-on-lead packagestaken during experiments in accordance with several embodiments of thetechnology.

DETAILED DESCRIPTION

Several embodiments of the present technology are described below withreference to processes of assembling microelectronic packages with leadframes and/or other suitable substrates. Typical microelectronicpackages include microelectronic circuits or components, thin-filmrecording heads, data storage elements, microfluidic devices, and othercomponents manufactured on microelectronic substrates. Microelectronicsubstrates can include semiconductor pieces (e.g., doped silicon wafersor gallium arsenide wafers), non-conductive pieces (e.g., variousceramic substrates), or conductive pieces (e.g., metal or metal alloy).Many of the details regarding certain embodiments are also describedbelow with reference to semiconductor dies. The term “semiconductor die”is used throughout to include a variety of articles of manufacture,including, for example, individual integrated circuit dies, imager dies,sensor dies, and/or dies having other semiconductor features.

Many specific details that relate to certain embodiments are set forthin the following text to provide a thorough understanding of theseembodiments. Several other embodiments can have configurations,components, and/or processes that are different from those describedbelow. A person skilled in the relevant art, therefore, will appreciatethat additional embodiments may be practiced without several of thedetails of the embodiments shown in FIGS. 1A-5B.

FIGS. 1A-1F are partially schematic, cross-sectional views of asemiconductor die and/or a portion of a lead frame undergoing a processto form a microelectronic package in accordance with embodiments of thetechnology. As shown in FIG. 1A, an initial stage of the process caninclude attaching a plurality of electrical couplers 104 to ball/bumpsites 102 of a semiconductor die 100, as indicated by the arrows 103.The electrical couplers 104 prior to attachment and/or plating are shownin phantom lines for clarity. In the illustrated embodiment, twoelectrical couplers 104 are shown for illustration purposes. In otherembodiments, one, three, or any other suitable numbers of electricalcouplers may be used.

The semiconductor die 100 can include any suitable type of integratedcircuit device. For example, in certain embodiments, the semiconductordie 100 can include a plurality of metal-oxide-semiconductorfield-effect transistors (“MOSFETs”), junction gate field-effecttransistors (“JFETs”), insulated gate bipolar transistors, capacitors,and/or other suitable electrical components. In other examples, thesemiconductor die 100 can include other suitable types of electricaland/or mechanical components.

In certain embodiments, the electrical couplers 104 can include solderballs attached to the ball/bump sites 102 using tack welding, partialreflow, and/or other suitable techniques. In other embodiments, theelectrical couplers 104 can include solder bumps plated onto and/orotherwise formed on the ball/bump sites 102. As used herein, the term“solder” generally refers to a fusible metal alloy with a melting pointin the range of about 90° C. to 450° C. Examples of a solder includealloys of at least some of copper (Cu), tin (Sn), lead (Pb), silver(Ag), zinc (Zn), and/or other suitable metals. In other embodiments, theelectrical couplers 104 can also include other suitable electricallyconductive couplers.

FIGS. 1B-1D illustrate stages of forming an attachment area 112 and anon-attachment area 113 on a lead frame 105 in accordance withembodiments of the technology. As shown in FIGS. 1B-1D, the lead frame105 can include a plurality of lead fingers 106. In the illustratedembodiment, a first lead finger 106 a is shown juxtaposed with a secondlead finger 106 b for illustration purposes. In other embodiments, thelead frame 105 can also include a die paddle (not shown), a dam bar (notshown), other number of lead fingers, and/or other suitable components.The lead fingers 106 can include a first surface 107 a opposite a secondsurface 107 b. As described in more detail later, the first surface 107a may be configured to interface with the semiconductor die 100 (FIG.1A), and the second surface 107 b may be configured to interface with anexternal device (e.g., a printed circuit board, not shown).

As shown in FIG. 1B, the process includes a deposition stage in which amasking material 108 is formed on the first surface 107 a of the leadfingers 106. In one embodiment, the masking material 108 can include aphotoresist deposited on the lead fingers 106 via a spin-on operationand/or other suitable techniques. In other embodiments, the photoresistcan include a tape of a dry film photoresist that may be laminated ontoand/or otherwise attached to the lead fingers 106. As used herein, theterm “photoresist” generally refers to a material that can be chemicallymodified when exposed to electromagnetic radiation. The term encompassesboth positive photoresist (i.e., soluble when activated by theelectromagnetic radiation) and negative photoresist (i.e., insolublewhen activated by the electromagnetic radiation). In other embodiments,the masking material 108 can also include hard rubber and/or othersuitable types of “hard” masking materials.

FIG. 1C illustrates a material removal stage of the process, in which aportion of the masking material 108 is removed to form a plurality ofopenings 110. In one embodiment, the material removal stage can includepatterning a photoresist based on a desired configuration usingphotolithography and/or other suitable techniques. In other embodiments,the openings 110 in the masking material 108 may also be formed usinglaser ablation, wet etching, dry etching, and/or other suitabletechniques.

As shown in FIG. 1D, another stage of the process includes forming theattachment area 112 and the non-attachment area 113 on the first surface107 a of the lead fingers 106. In the illustrated embodiment, formingthe attachment area 112 (or solder wetting pads) includes depositing awetting material 111 into the openings 110 of the masking material 108.The wetting material 111 can include silver (Ag), a nickel (Ni)/gold(Au) alloy, and/or other suitable metal or metal alloys that exhibitfirst wetting characteristics with regard to the electrical couplers 104(FIG. 1A) during reflow. As used herein, wetting characteristics arerepresented by a wetting contact angle. For example, in certainembodiments, the wetting contact angle between the wetting material 111and the electrical couplers 104 during reflow can be less than about90°, indicating generally wettable characteristics. In otherembodiments, the wetting contact angle can be less than about 60°, about45°, about 30°, and/or other suitable values.

After forming the attachment area 112, the process can include removingthe masking material 108 (shown in phantom lines for clarity) and anyexcess wetting material 111. Subsequently, the exposed portions of thefirst surface 107 a can be treated to form the non-attachment area 113that has second wetting characteristics. In one embodiment, the exposedportions of the first surface 107 a may be oxidized by contacting anoxidizing chemical solution (e.g., sulfuric acid, nitric acid,hydrochloric acid, and/or a combination thereof), heating the leadfingers 106 in air, by contacting with oxygen plasma, and/or by usingother suitable techniques. In other embodiments, the exposed portions ofthe first surface 107 a may be treated using other suitable surfacetreatment techniques.

After surface treatment, the non-attachment area 113 can be generallynon-wettable to the electrical couplers 104 (FIG. 1A) during reflow. Forexample, in one embodiment, a contact angle between the non-attachmentarea 113 and the electrical couplers 104 during reflow can be greaterthan about 90°, indicating generally non-wettable characteristics. Inother examples, the non-wetting contact angle can be greater than about120°, about 135°, and/or other suitable values.

FIG. 1E illustrates another stage of the process in which thesemiconductor die 100 is attached to the lead frame 105 with theelectrical couplers 104 generally aligned and in contact with theattachment area 112 of the lead fingers 106. The lead frame 105 with theattached semiconductor die 100 can then be reflowed, for example, in areflow chamber (not shown), and heat (as represented by the arrows 116)and/or other suitable forms of energy can be applied. As a result, theelectrical couplers 104 can be at least partially melted in order tojoin the lead frame 105 and the semiconductor die 100 together whensubsequently cooled.

It has been observed that the lead fingers 106 with the attachment area112 and the non-attachment area 113 can enable a controllable collapseof the electrical couplers 104 during the reflow operation without usinga solder mask. As a result, the risk of various structural and/orelectrical damages to the resulting microelectronic package may bereduced and/or avoided. As discussed above, the attachment area 112 isgenerally wettable while the non-attachment area 113 is generallynon-wettable to the electrical couplers 104. Without being bound bytheory, it is believed that the wettability differential between theattachment area 112 and the non-attachment area 113 can at least limitor substantially eliminate migration or spreading of the reflowedelectrical couplers 104. It is believed that the reflowed electricalcouplers 104 may not readily bond to the non-attachment area 113 due toa lack of surface contact. As a result, the reflowed electrical couplers104 tend to be confined in the attachment area 112.

In certain embodiments, a wettability differential between theattachment area 112 and the non-attachment area 113 may be adjustedbased on a target degree of migration of the reflowed electricalcouplers 104. In general, it is believed that the larger the wettabilitydifferential, the smaller the degree of migration, and vice versa. Thus,if a small degree of migration is desired, a large wettabilitydifferential (e.g., a contact angle difference of greater than about20°, about 30°, or about 40°) may be used. If a large degree ofmigration may be tolerated, a small wettability differential (e.g., acontact angle difference of less than about 15°, about 10°, or about 5°)may be used.

Subsequent to reflow, the process can also include various additionalprocessing stages. For example, as shown in FIG. 1F, an underfillmaterial 117 may be deposited between the semiconductor die 100 and thelead fingers 106. The underfill material 117 may at least partiallyencapsulate the electrical couplers 104. In other embodiments, theunderfill material 117 may be omitted.

The semiconductor die 100 and the lead frame 105 can also beencapsulated by an encapsulant 120 (e.g., an epoxy or other types ofmolding compounds). In the illustrated embodiment, the semiconductor die100 is substantially encapsulated in the encapsulant 120. The leadfingers 106 are partially encapsulated in the encapsulant 120 with thesecond surfaces 107 b exposed for interconnecting to external devices(not shown). In other embodiments, the lead frame 105 may include leadfingers 106 that extend beyond the encapsulant 120. In furtherembodiments, the semiconductor die 100 and the lead frame 105 may beencapsulated in other suitable configurations.

Even though the foregoing process includes forming the attachment area112 and the non-attachment area 113 by depositing the wetting material111 and surface treating the lead fingers 106, in other embodiments,forming the attachment area 112 and the non-attachment area 113 mayinclude other processing operations. For example, FIGS. 2A-2E illustrateanother process that does not include depositing a wetting material.

As shown in FIG. 2A, an initial stage of the process can includedepositing the masking material 108 on the first surface 107 a of thelead fingers 106. As shown in FIGS. 2B and 2C, another stage of theprocess can include patterning the masking material 108 to define acovered portion 109 a and an exposed portion 109 b of the first surface107 a of the lead fingers 106. The covered portion 109 a generallycorresponds to the attachment area 112, and the exposed portion 109 bgenerally corresponds to the non-attachment area 113. In the illustratedembodiment, the covered portion 109 a is illustratively shown to have agenerally circular shape. In other embodiments, the covered portion 109a may have a rectangular, an oval, a trapezoidal, and/or other suitableshapes.

The exposed portion 109 b may then be treated to achieve the targetwettability characteristics while the covered portion 109 a is protectedfrom the treatment by the remaining masking material 108. In certainembodiments, the exposed portion 109 b may be treated in a generallysimilar manner, as described above with reference to FIG. 1D. In otherembodiments, the exposed portion 109 b may be treated using othersuitable techniques such that the exposed portion 109 b is generallynon-wettable to the electrical couplers 104 (FIG. 1A) during reflow.

Similar to the process described above with reference to FIGS. 1A-1F,the exposed area 109 b of the lead fingers 106 may be treated based on atarget wettability differential between the attachment area 112 and thenon-attachment area 113. For example, in one embodiment, the lead frame105 can include copper (Cu) lead fingers 106. As a result, thewettability of the reflowed electric coupler 104 (FIG. 1A) to the copperlead fingers 106 may be readily determined. Thus, a target wettabilityfor the non-attachment area 113 may be derived based on the wettabilityof the reflowed electric coupler 104 (FIG. 1A) to the copper leadfingers 106 and the target wettability differential. Based on the targetwettability for the non-attachment area 113, suitable techniques and/oroperation parameters may be selected (e.g., oxygen plasma treatment,heating, etc.) to achieve the target wettability for the non-attachmentarea 113.

As shown in FIGS. 2D and 2E, another stage of the process can includeremoving the remaining masking material 108 (FIGS. 2B and 2C) to exposethe attachment area 112. Subsequently, the lead frame 105 can undergoother suitable processing stages to form the microelectronic package, asdescribed in more detail with reference to FIGS. 1E and 1F.

FIGS. 3A-3C illustrate another process of forming the attachment area112 and the non-attachment area 113 using screen printing. As shown inFIG. 3A, an initial stage of the process can include placing a stencil132 proximate the lead fingers 106. The stencil 132 can includeapertures 134 generally corresponding to the attachment area 112.Subsequently, as shown in FIG. 3B, the wetting material 111 (representedby the arrows 111 in FIG. 3A) may be sprayed, printed, and/or otherwiseformed on the first surface 107 a of the lead fingers 106 through theapertures 134 of the stencil 132.

As shown in FIG. 3C, the process can then include removing the stencil132 (FIG. 3B) from the first surface 107 a of the lead fingers 106. Theexposed portion of the first surface 107 a may then optionally undergosurface treatment, as described above with reference to FIG. 1D.Subsequently, the lead frame 105 can undergo other suitable processingstages to form the microelectronic package, as described in more detailwith reference to FIGS. 1E and 1F.

FIGS. 4A and 4B illustrate another process for forming the attachmentarea 112 and the non-attachment area 113 using a preform. As shown inFIG. 4A, the process can include attaching a preform 150 to the firstsurface 107 a of the lead fingers 106. In the illustrated embodiment,the preform 150 includes a laminated structure with an interface layer152 and an adhesive layer 156. The interface layer 152 can include afirst portion 154 that is wettable to the reflowed electrical couplers104 (FIG. 1A) and a second portion 158 that is generally non-wettable tothe reflowed electrical couplers 104. For example, the first portion 154can include silver (Ag), and the second portion 158 can include copperoxide (Cu_(x)O). As a result, the first portion 154 and the secondportion 158 generally correspond to the attachment area 112 and thenon-attachment area 113.

As shown in FIG. 4B, the lead frame 105 with the attached preform 150may then be optionally cured and/or undergo other suitable processingbefore being attached to the semiconductor die 100, as described in moredetail with reference to FIGS. 1E and 1F. Even though the preform 150 isshown in FIGS. 4A and 4B as having the adhesive layer 156 pre-attached,in other embodiments, the preform 150 may include the interface layer152 carried by an optional backing layer (not shown) without theadhesive layer 156. Instead, an adhesive (not shown) may be applied toat least one of the preform 150 and the lead frame 105 before attachmentthereof.

Experiments of assembling chip-on-lead packages were conducted. FIGS. 5Aand 5B are cross-sectional images of chip-on-lead packages taken duringsuch experiments. In a first experiment, a semiconductor die 200 withattached solder balls 204 was placed in contact with a lead frame 205with copper lead fingers 206 that have generally the same regionalwettability. The semiconductor die 200 and the lead frame 205 weresubsequently reflowed. As clearly shown in FIG. 5A, the solder balls 206uncontrollably collapsed during reflow and came in contact with eachother.

In a second experiment, a semiconductor die 300 with an attached solderball 304 was placed in contact with a lead frame 305 with a lead finger306 processed according to several embodiments of the process describedabove with reference to FIGS. 1A-1E. The lead finger 306 included acontact pad 312 containing silver (Ag) on a portion of the top surfaceof the lead finger 306. The semiconductor die 300 and the lead frame 305were subsequently reflowed. As clearly shown in FIG. 5B, the solder ball304 substantially retained its shape and did not significantly migrateaway from the contact pad 312.

From the foregoing, it will be appreciated that specific embodiments ofthe technology have been described herein for purposes of illustration,but that various modifications may be made without deviating from thetechnology. Many of the elements of one embodiment may be combined withother embodiments in addition to or in lieu of the elements of the otherembodiments. Accordingly, the technology is not limited except as by theappended claims.

I/we claim:
 1. A method for fabricating a semiconductor assembly,comprising: forming an attachment area and a non-attachment area on alead finger of a lead frame, the attachment area being more wettable toa solder ball than the non-attachment area during reflow; contacting asolder ball carried by a semiconductor die with the attachment area ofthe lead finger; reflowing the solder ball while the solder ball is incontact with the attachment area of the lead finger; and controllablycollapsing the solder ball to establish an electrical connection betweenthe semiconductor die and the lead finger of the lead frame; and whereincontrollably collapsing includes: preventing the solder ball frommigrating away from the silver in the attachment area with the copperoxide in the non-attachment area.
 2. The method of claim 1 furthercomprising: attaching the solder ball to a contact pad of thesemiconductor die; forming the attachment area and the non-attachmentarea includes: depositing a photoresist on a surface of the lead fingercontaining copper (Cu); patterning the photoresist to form an openingcorresponding to the attachment area, the opening exposing a firstportion of the surface of the lead finger, a second portion of thesurface being covered by the photoresist; depositing silver (Ag) on thefirst portion of the surface of the lead finger through the opening inthe photoresist; removing the photoresist from the second portion of thesurface of the lead finger having the deposited silver; and oxidizingthe second portion of the surface of the lead finger thereby formingcopper oxide (Cu_(x)O) attaching the solder ball to a contact pad of thesemiconductor die; forming the attachment area and the non-attachmentarea includes: depositing a photoresist on a surface of the lead fingercontaining copper (Cu); patterning the photoresist to form an openingcorresponding to the attachment area, the opening exposing a firstportion of the surface of the lead finger, a second portion of thesurface being covered by the photoresist; depositing silver (Ag) on thefirst portion of the surface of the lead finger through the opening inthe photoresist; removing the photoresist from the second portion of thesurface of the lead finger having the deposited silver; and oxidizingthe second portion of the surface of the lead finger thereby formingcopper oxide (Cu_(x)O)
 3. The method of claim 1, further comprising:attaching the solder ball to a contact pad of the semiconductor dieprior to contacting the solder ball with the attachment area of the leadfinger; and at least partially encapsulating the semiconductor die, thesolder ball, and the lead finger with an encapsulant.
 4. The method ofclaim 1 wherein forming the attachment area and the non-attachment areaincludes: depositing a masking material on a surface of the lead finger;patterning the masking material to form an opening corresponding to theattachment area, the opening exposing a first portion of the surface ofthe lead finger, wherein a second portion of the surface is covered bythe masking material; depositing a wetting material on the first portionof the surface of the lead finger through the opening in the maskingmaterial; removing the masking material from the surface of the leadfinger; and treating the second portion of the surface of the leadfinger such that the second portion is less wettable to the solder ballduring reflow than the first portion.
 5. The method of claim 1 whereinforming the attachment area and the non-attachment area includes:depositing a photoresist on a surface of the lead finger containingcopper (Cu); patterning the photoresist to form an opening correspondingto the attachment area, the opening exposing a first portion of thesurface of the lead finger, wherein a second portion of the surface iscovered by the photoresist; depositing silver (Ag) on the first portionof the surface of the lead finger through the opening in thephotoresist; removing the photoresist from the second portion of thesurface of the lead finger; and oxidizing the second portion of thesurface of the lead finger thereby forming copper oxide (Cu_(x)O). 6.The method of claim 1 wherein forming the attachment area and thenon-attachment area includes: depositing a wetting material on a firstportion of the lead finger; and treating a second portion of the leadfinger such that the second portion is less wettable to the solder ballduring reflow than the first portion.
 7. The method of claim 1 whereinforming the attachment area and the non-attachment area includes:depositing a masking material on a surface of the lead finger; removinga portion of the masking material, a remaining portion of the maskingmaterial covering a first portion of the surface of the lead finger,wherein a second portion of the surface of the lead finger is exposedthrough the masking material; treating the second portion of the surfaceof the lead finger such that the second portion is less wettable to thesolder ball during reflow than the first portion; and thereafter,removing the remaining portion of the masking material from the leadfinger.
 8. The method of claim 7 wherein treating the second portionincludes oxidizing the second portion of the surface of the lead finger.9. The method of claim 1 wherein forming the attachment area and thenon-attachment area includes: placing a stencil proximate to a surfaceof the lead finger, the stencil having an opening generallycorresponding to the attachment area; and printing a wetting materialonto the surface of the lead finger through the opening in the stencil.10. The method of claim 1 wherein forming the attachment area and thenon-attachment area includes attaching a preform onto the lead fingerwith an adhesive, the preform having a first portion generallycorresponding to the attachment area and a second portion generallycorresponding to the non-attachment area.
 11. The method of claim 1wherein controllably collapsing includes preventing the solder ball frommigrating away from the attachment area with the non-attachment area.12. A method for fabricating a semiconductor assembly, comprising:forming an attachment area and a non-attachment area on a lead finger ofa lead frame; contacting a solder ball carried by a semiconductor diewith the attachment area of the lead finger; reflowing the solder ballwhile the solder ball is in contact with the attachment area of the leadfinger; and preventing the solder ball from migrating away from theattachment area toward the non-attachment area; and wherein: theattachment area has a first wettability to the solder ball duringreflow; the non-attachment area has a second wettability to the solderball during reflow; and preventing the solder ball from migratingincludes treating at least one of the attachment area and thenon-attachment area such that the first wettability is less than thesecond wettability for a target amount; and the method further includesdetermining the target amount based on a target degree of migration ofthe solder ball from the attachment area during reflow; and preventingthe solder ball from migrating includes at least one of (a) depositingsilver (Ag) onto a first portion of the lead finger and (b) oxidizing asecond portion of the lead finger.
 13. The method of claim 12 furthercomprising: determining the target amount based on a target degree ofmigration of the solder ball from the attachment area during reflow. 14.The method of claim 12 wherein preventing the solder ball from migratingincludes at least one of (a) depositing silver (Ag) onto a first portionof the lead finger and (b) oxidizing a second portion of the leadfinger.
 15. The method of claim 13 wherein preventing the solder ballfrom migrating includes at least one of (a) depositing silver (Ag) ontoa first portion of the lead finger and (b) oxidizing a second portion ofthe lead finger.